The present invention relates to an air-conditioning system having a refrigeration circuit. More particularly, the present invention pertains to a method for controlling the displacement of a variable displacement compressor and to a control valve used in a variable displacement compressor.
A refrigeration circuit of a typical vehicle air-conditioning system includes a condenser, an expansion valve, which functions as a decompression device, an evaporator and a compressor. The compressor draws refrigerant gas from the evaporator and compresses the gas. The compressor then discharges the gas to the condenser. The evaporator performs heat exchange between the refrigerant in the circuit and air in the passenger compartment. Heat from air that flows about the evaporator is transferred to the refrigerant flowing through the evaporator in accordance with the thermal load or the cooling load. The pressure of the refrigerant gas at the outlet of the evaporator represents the magnitude of the thermal load.
A vehicle variable displacement swash plate type compressor has a displacement control mechanism for setting the pressure (suction pressure Ps) in the vicinity of the outlet of the evaporator to a predetermined target suction pressure. The mechanism adjusts the compressor displacement by changing the inclination angle of the swash plate such that the flow rate of refrigerant corresponds to the cooling load. To control the displacement, a control valve is used. The control valve includes a pressure sensing member, which is a bellows or a diaphragm. The pressure sensing member detects the suction pressure Ps. A valve opening is adjusted in accordance with the displacement of the pressure sensing member, which changes the pressure in a crank chamber, or crank pressure Pc.
A simple control valve that controls a single target suction pressure cannot control the air conditioning performance accurately. Therefore, an electromagnetic control valve that changes a target suction pressure in accordance with an external current has been introduced. Such a control valve includes an electromagnetic actuator such as a solenoid. The actuator changes force acting on a pressure sensing member in accordance with an external current to adjust a target suction pressure.
A typical on-vehicle compressor is driven by an engine. A compressor is one of the devices that consume most of the power (or the torque) of an engine. Therefore, when the load on the engine is great, for example, when the vehicle is accelerating or moving uphill, the compressor displacement is minimized to reduce the engine load. Specifically, the value of current supplied to the electromagnetic control valve is controlled for setting the target suction pressure to a relatively great value. Accordingly, to increase the actual suction pressure toward the target suction pressure, the control valve operates such that the compressor displacement is minimized.
A graph of FIG. 22 illustrates the relationship between a suction pressure Ps and the displacement Vc of a compressor. The relationship is represented by multiple lines in accordance with the thermal load in an evaporator. Thus, if a level Ps1 is set as a target suction pressure Pset, the actual displacement Vc varies in a certain range (xcex94Vc in FIG. 22) due to the thermal load. For example, when an excessive thermal load is applied to the evaporator, an increase of the target suction pressure Pset may not decrease the engine load. That is, even if the target suction pressure Pset is raised, the compressor displacement Vc will not be lowered to a level that reduces the engine load unless the thermal load on the evaporator is relatively small.
The suction pressure Ps represents the thermal load on an evaporator. The method for controlling the displacement of a variable displacement compressor based on the suction pressure Ps is appropriate for maintaining the temperature in a vehicle compartment at a comfortable level. However, to quickly decrease the displacement, the displacement control based only on the suction pressure Ps is not always appropriate. For example, the displacement control based on the suction pressure Ps is not suitable for the above described displacement limiting control procedure, in which the displacement must be quickly decreased to secure the engine power.
Accordingly, it is an objective of the present invention to provide an air-conditioning system that rapidly changes the displacement of a compressor. Particularly, the objective of the present invention is to provide a method for controlling a variable displacement compressor and a control valve of a variable displacement compressor that stabilizes the temperature of a vehicle passenger compartment and be quickly changed to secure the engine power.
To achieve the above objective, the present invention provides an air-conditioning system having a refrigerant circuit. The circuit includes a condenser, a decompression device, an evaporator and a variable displacement compressor. The system comprises a pressure difference detector for detecting the pressure difference between two pressure monitoring points located on the refrigerant circuit, and means for controlling the displacement of the compressor in accordance with the pressure difference detected by the pressure difference detector.
To achieve the above objective, the present invention also provides a method for controlling the displacement of a variable displacement compressor in a refrigerant circuit of a vehicle air-conditioning system. The method comprises selecting a control mode from a plurality of modes including a normal displacement control mode and a specific displacement control mode, changing the displacement of the compressor such that the pressure difference between two pressure monitoring points, which are located in the refrigerant circuit, approaches a target pressure difference that reflects the temperature of a passenger compartment when the normal displacement control mode is selected, and controlling the compressor to have a predetermined displacement when the specific displacement control is selected.
To achieve the above objective, the present invention also provides a control valve used for a variable displacement compressor. The compressor is a part of a refrigerant circuit. The compressor includes a crank chamber, a drive plate accommodated in the crank chamber, a supply passage for connecting a discharge pressure zone to the crank chamber, and a bleed passage for connecting a suction pressure zone to the crank chamber. The inclination angle of the drive plate varies in accordance with the pressure in the crank chamber thereby controlling the displacement of the compressor. The control valve comprises a valve housing. A valve chamber is defined in the valve housing to form part of the supply passage or the bleed passage. A movable valve body is located in the valve chamber to adjust opening size of the supply passage or the bleed passage. A pressure difference detector detects the pressure difference between two pressure monitoring points located in the refrigerant circuit. The position of the valve body is affected by based on a force produced by the detected pressure difference. An actuator applies a force to the pressure difference detector, wherein the actuator changes a target pressure difference according to the external command.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.